arXiv subjects
Publications and source records attributed to Nicola Poccia.
We respond to P. Ao's comment in arXiv:1907.09263, which suggests that vortex many-body effects are the origin of Hall sign reversal in few-unit-cell thick Bi-2212 cuprate crystals (Phys. Rev. Lett. 122, 247001 (2019)). Our experimental results are incompatible with the theoretical predictions detailed in Ao's comment.
In several strongly correlated electron systems, defects, charge and local lattice distortions are found to show complex inhomogeneous spatial distributions. There is growing evidence that such inhomogeneity plays a fundamental role in unique functionality of quantum complex materials. La1.72Sr0.28NiO4 is a prototypical strongly correlated material showing spin striped order associated with lattice and charge modulations. In this work we present the spatial distribution of the spin organization by applying micro X-ray diffraction to La1.72Sr0.28NiO4, mapping the spin-density-wave order below the 120K onset temperature. We find that the spin-density-wave order shows the formation of nanoscale puddles with large spatial fluctuations. The nano-puddle density changes on the microscopic scale forming a multiscale phase separation extending from nanoscale to micron scale with scale-free distribution. Indeed spin-density-wave striped puddles are disconnected by spatial regions with different stripe orientation or negligible spin-density-wave order. The present work highlights the complex nanoscale phase separation of spin stripes in nickelate perovskites and opens the question of the energetics at domain interfaces
We fabricate van der Waals heterostructure devices using few unit cell thick Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ for magnetotransport measurements. The superconducting transition temperature and carrier density in atomically thin samples can be maintained to close to that of the bulk samples. As in the bulk sample, the sign of the Hall conductivity is found to be opposite to the normal state near the transition temperature but with a drastic enlargement of the region of Hall sign reversal in the temperature-magnetic field phase diagram as the thickness of samples decreases. Quantitative analysis of the Hall sign reversal based on the excess charge density in the vortex core and superconducting fluctuations suggests a renormalized superconducting gap in atomically thin samples at the 2-dimensional limit.
Ultrastructural fluctuations at nanoscale are fundamental to assess properties and functionalities of advanced out-of-equilibrium materials. We have taken myelin as a model of supramolecular assembly in out-of-equilibrium living matter. Myelin sheath is a simple stable multi-lamellar structure of high relevance and impact in biomedicine. Although it is known that myelin has a quasi-crystalline ultrastructure there is no information on its fluctuations at nanoscale in different states due to limitations of the available standard techniques. To overcome these limitations, we have used Scanning micro X-ray Diffraction, which is a non-invasive probe of both reciprocal and real space to visualize statistical fluctuations of myelin order of the sciatic nerve of Xenopus Laevis. The results show that the ultrastructure period of the myelin is stabilized by large anti-correlated fluctuations at nanoscale, between hydrophobic and hydrophilic layers. The ratio between the total thickness of hydrophilic and hydrophobic layers defines the conformational parameter, which describes the different states of myelin. Our key result is that myelin in its out-of-equilibrium functional state fluctuates point-to-point between different conformations showing a correlated disorder described by a Levy distribution. As the system approaches the thermodynamic equilibrium in an aged state the disorder loses its correlation degree and the structural fluctuation distribution changes to Gaussian. In a denatured state at low pH, it changes to a completely disordered stage. Our results clarify also the degradation mechanism in biological systems by associating these states with variation of the ultrastructural dynamic fluctuations at nanoscale.
While the ultrastructure of the myelin has been considered to be a quasi-crystalline stable system, nowadays its multiscale complex dynamics appears to play a key role for its functionality, degeneration and repair processes following neurological diseases and trauma. In this work, we have investigated the axons interactions associated to the nerve functionality, measuring the spatial distribution of the orientational fluctuations of axons in a Xenopus Laevis sciatic nerve. At this aim, we have used Scanning micro X-ray Diffraction (SmXRD), a non-invasive already applied to other heterogeneous systems presenting complex geometries from microscale to nanoscale. We have found that the orientational spatial fluctuations of fresh axons show a correlated disorder described by Levy flight distribution. Thus, we have studied how this correlated disorder evolves during the degeneration of the nerve. Our results show that the spatial distribution of axons orientational fluctuations in unfresh, aged nerve loose the correlated disorder assuming a randomly disordered behaviour. This work allows a deeper understanding of nerve states and paves the way to study other materials and biomaterials with the same technique to detect and to characterize their states and supramolecular structure, associated with dynamic structural changes at the nanoscale and mesoscale.
It is now well established that superconductivity in cuprates competes with charge modulations giving electronic phase separation at a nanoscale. More specifically, superconducting electronic current takes root in the available free space left by electronic charge ordered domains, called charge density wave (CDW) puddles. This means that CDW domain arrangement plays a fundamental role in the mechanism of high temperature superconductivity in cuprates. Here we report about the possibility of controlling the population and spatial organization of the charge density wave puddles in a single crystal La2CuO4+y through X - ray illumination and thermal treatments. We apply a pump - probe method based on X - ray illumination as pump and X - ray diffraction as a probe setting a writing and reading procedure of CDW puddles. Our findings are expected to allow new routes for advanced design and manipulation of superconducting pathways in new electronics.
The dynamic Mott insulator-to-metal transition (DMT) is key to many intriguing phenomena in condensed matter physics yet it remains nearly unexplored. The cleanest way to observe DMT, without the interference from disorder and other effects inherent to electronic and atomic systems, is to employ the vortex Mott states formed by superconducting vortices in a regular array of pinning sites. The applied electric current delocalizes vortices and drives the dynamic vortex Mott transition. Here we report the critical behavior of the vortex system as it crosses the DMT line, driven by either current or temperature. We find universal scaling with respect to both, expressed by the same scaling function and characterized by a single critical exponent coinciding with the exponent for the thermodynamic Mott transition. We develop a theory for the DMT based on the parity reflection-time reversal (PT) symmetry breaking formalism and find that the nonequilibrium-induced Mott transition has the same critical behavior as thermal Mott transition. Our findings demonstrate the existence of physical systems in which the effect of nonequilibrium drive is to generate effective temperature and hence the transition belonging in the thermal universality class. We establish PT symmetry-breaking as a universal mechanism for out-of-equilibrium phase transitions.
Multiple functional ionic and electronic orders are observed in high temperature superconducting cuprates. The charge density wave order is one of them and it is spatially localized in spatial regions of the material. It is also known that the oxygen interstitials introduced by chemical intercalation self-organize in different oxygen rich regions corresponding with hole rich regions in the Cu$O_2$ layers left empty by the charge density wave order domains. However, what happens in between these two order is not known, and neither there is a method to control this spatial separation. Here we demonstrate by using scanning nano X-ray diffraction, that dislocations or grain boundaries in the material can act as boundary between charge density wave and oxygen rich phases in a optimally doped La$_2$CuO$_4$$_+$$_y$ high temperature superconductor. Dislocations can be used therefore to control the anti-correlation of the charge density wave order with the oxygen interstitials in specific portion of the material.
We report the experimental results of temperature dependent polarized As K-edge extended x-ray absorption fine structure (EXAFS) of LaFe1-xCoxAsO (x=0.0 and 0.11) single-crystals. By aligning the Fe-As bond direction in the direction of the x-ray beam polarization we have been able to identify an anomaly in the Fe-As bond correlations at the tetragonal to orthorhombic transition at 150K, while previous investigations with standard unpolarized EXAFS of undoped LaFeAsO powder samples were not able to detect any such anomaly. Using our approach we have been able to identify in the superconducting doped sample, LaFe0.89Co0.11AsO, a broad anomaly around 60 K. The low temperature anomaly has good correlations with the temperature dependence of several properties like resistivity, magnetic susceptibility, linear thermal expansion, etc indicating the emergence of the dynamical oscillations of the Fe - As pairs
Recently, x-ray illumination, using synchrotron radiation, has been used to manipulate defects, stimulate self-organization and to probe their structure. Here we explore a method of defect-engineering low-dimensional systems using focused laboratory-scale X-ray sources. We demonstrate an irreversible change in the conducting properties of the 2-dimensional electron gas at the interface between the complex oxide materials LaAlO3 and SrTiO3 by X-ray irradiation. The electrical resistance is monitored during exposure as the irradiated regions are driven into a high resistance state. Our results suggest attention shall be paid on electronic structure modification in X-ray spectroscopic studies and highlight large-area defect manipulation and direct device patterning as possible new fields of application for focused laboratory X-ray sources.
The arrested nanoscale phase separation in a two-band Hubbard model for strongly correlated charge carriers is shown to occur in a particular range in vicinity of the topological Lifshitz transition, where the Fermi energy crosses the bottom of the narrow band and a new sheet of the Fermi surface related to the charge carriers of the second band comes into play. We determine the phase separation diagram of this two-band Hubbard model as a function of two variables, the charge carrier density and the energy shift between the chemical potential and the bottom of the second band. In this phase diagram, we first determine a line of quantum critical points for the Lifshitz transition and find criteria for the electronic phase separation resulting in an inhomogeneous charge distribution. Finally, we identify the critical point in presence of a variable long-range Coulomb interaction where the scale invariance of the coexisting phases with different charge densities appears. We argue that this point is relevant for the regime of scale invariance of the nanoscale phase separation in cuprates like it was first observed in La$_2$CuO$_{4.1}$.
Atomically sharp oxide heterostructures often exhibit unusual physical properties that are absent in the constituent bulk materials. The interplay between electrostatic boundary conditions, strain and dimensionality in ultrathin epitaxial films can result in monolayer-scale transitions in electronic or magnetic properties. Here we report an atomically sharp antiferromagnetic-to-ferromagnetic phase transition when atomically growing polar antiferromagnetic LaMnO3 (001) films on SrTiO3 substrates. For a thickness of five unit cells or less, the films are antiferromagnetic, but for six unit cells or more, the LaMnO3 film undergoes a phase transition to a ferromagnetic state over its entire area, which is visualized by scanning superconducting quantum interference device microscopy. The transition is explained in terms of electronic reconstruction originating from the polar nature of the LaMnO3 (001) films. Our results demonstrate how new emergent functionalities can be visualized and engineered in atomically thick oxide films at the atomic level.
We investigate magnetoresistance of a square array of superconducting islands placed on a normal metal, which offers a unique tunable laboratory for realizing and exploring quantum many-body systems and their dynamics. A vortex Mott insulator where magnetic field-induced vortices are frozen in the dimples of the egg crate potential by their strong repulsion interaction is discovered. We find an insulator-to-metal transition driven by the applied electric current and determine critical exponents that exhibit striking similarity with the common thermodynamic liquid-gas transition. A simple and straightforward quantum mechanical picture is proposed that describes both tunneling dynamics in the deep insulating state and the observed scaling behavior in the vicinity of the critical point. Our findings offer a comprehensive description of dynamic Mott critical behavior and establish a deep connection between equilibrium and nonequilibrium phase transitions.
Degradation of the myelin sheath is a common pathology underlying demyelinating neurological diseases from Multiple Sclerosis to Leukodistrophies. Although large malformations of myelin ultrastructure in the advanced stages of Wallerian degradation is known, its subtle structural variations at early stages of demyelination remains poorly characterized. This is partly due to the lack of suitable and non-invasive experimental probes possessing sufficient resolution to detect the degradation. Here we report the feasibility of the application of an innovative non-invasive local structure experimental approach for imaging the changes of statistical structural fluctuations in the first stage of myelin degeneration. Scanning micro X-ray diffraction, using advances in synchrotron x-ray beam focusing, fast data collection, paired with spatial statistical analysis, has been used to unveil temporal changes in the myelin structure of dissected nerves following extraction of the Xenopus laevis sciatic nerve. The early myelin degeneration is a specific ordered compacted phase preceding the swollen myelin phase of Wallerian degradation. Our demonstration of the feasibility of the statistical analysis of SmXRD measurements using biological tissue paves the way for further structural investigations of degradation and death of neurons and other cells and tissues in diverse pathological states where nanoscale structural changes may be uncovered.
The simplest cuprate superconductor La2CuO4+y with mobile oxygen interstitials exhibits a clear phase separation, but only recently a bulk multiscale structural phase separation has been observed by using scanning micro X-ray diffraction. Here we get further information on the structural phase separation, using local probe X-ray absorption near edge structure. The spatial distribution of superconducting units is a key parameter controlling percolative superconductivity in complex matter with dispersed superconducting units. These oxides form super-molecular architectures made of superconducting atomic monolayers intercalated by spacers. Oxygen interstitials enter into the rocksalt La2O2+y spacer layers forming oxygen interstitials rich puddles and poor puddles. Their spatial distribution has been determined by using scanning La L3-edge micro X-ray absorption near edge structure. Percolating networks of oxygen rich puddles are observed in different micrometer size portions of the crystals. Moreover, the complex surface resistivity shows two jumps associated to the onset of intra-puddle and inter-puddles percolative superconductivity. The similarity of oxygen doped La2CuO4+y, with the well established phase separation in iron selenide superconductors is also discussed.
Despite intensive research a physical explanation of high Tc superconductors remains elusive. One reason for this is that these materials have generally a very complex structure making useless theoretical models for a homogeneous system. Little is known on the control of the critical temperature by the space disposition of defects because of lack of suitable experimental probes. X-ray diffraction and neutron scattering experiments used to investigate y oxygen dopants in YBa2Cu3O 6+y lack of spatial resolution. Here we report the spatial imaging of dopants distribution in-homogeneity in YBa2Cu3O6.67 using scanning nano X-ray diffraction. By changing the X-ray beam size from 1 micron to 300 nm of diameter, the lattice inhomogeneity increases. The ordered oxygen puddles size distribution vary between 6-8 nm using 1x1 μm2 beam, while it is between 5-12 nm with a fat tail using the 300x300 nm2 beam. The increased inhomogeneity at the nanoscale points toward an intrinsic granular complexity.
While it is known that the nature and the arrangement of defects in complex oxides have an impact on the material functionalities little is known on control of superconductivity by oxygen interstitial organization in cuprates. Here we report direct compelling evidence for the control of Tc, by manipulation of the superconducting granular networks of nanoscale puddles, made of ordered oxygen stripes, in a single crystal of YBa2Cu3O6.5+y with average formal hole doping p close to 1/8. Upon thermal treatments we were able to switch from a first network of oxygen defects striped puddles with OVIII modulation (qOVIII(a*)=(h+3/8,k,0) and qOVIII(a*)=(h+5/8,k,0)), to second network characterized by OXVI modulation (qOXVI(a*)=(h+7/16,k,0) and qOXVI(a*)=(h+9/16,k,0)), and finally to a third network with puddles of OV periodicity (qOV(a*)=(4/10,1,0) and qOV(a*)=(6/10,1,0)). We map the microscopic spatial evolution of the out of plane OVIII, OXVI and OV puddles nano-size distribution via scanning micro-diffraction measurements. In particular, we calculated the number of oxygen chains (n) and the charge density (holes concentration p) inside each puddle, analyzing areas of 160x80 μm2, and recording 12800 diffraction patterns to reconstruct each spatial map. The high spatial inhomogeneity shown by all the reconstructed spatial maps reflects the intrinsic granular structure that characterizes cuprates and iron-chalcogenides, disclosing the presence of several complex networks of coexisting superconducting domains with different lattice modulations, charge density and different gaps like in the proposed multi-gaps scenario called superstripes.
Oxygen chain fragments are known to appear at the insulator to superconductor transition (SIT) in YBa2Cu3O6+y. However the self organization and the size distribution of oxygen chain fragments is not known. Here we contribute to fill this gap, using scanning micro X ray diffraction which is a novel imaging method based on advances in focusing synchrotron radiation beam. This novel approach allows us to probe both real-space and k-space of a high-quality YBa2Cu3O6.33 single crystals with Tc=7K. We report compelling evidence for nanoscale striped puddles, with Ortho-II structure, made of chain fragments in the basal Cu(1) plane with local oxygen concentration 0.5. The size of the Ortho-II puddles spans a range between 2 and 9 nanometers. The real space imaging of Ortho-II puddles granular network shows that superconductivity, at low hole-doping regime, occurs in a network of nanoscale oxygen ordered patches, interspersed with oxygen depleted regions. The manipulation by thermal treatments of the striped Ortho II puddles has been investigated focusing on the spontaneous symmetry breaking near the order to disorder phase transition at 350 K.